In an engine system, an exhaust gas recirculation (EGR) system decreases a combustion temperature of an engine by re-circulating part of exhaust gas, discharged from the engine, to a combustion chamber of the engine, thereby reducing the amount of nitrogen oxide NOx to be generated. The EGR system includes an exhaust gas recirculation line branched off from an exhaust line of the engine and extended to an intake line of the engine, and an EGR valve configured to regulate the amount of exhaust gas entering from the exhaust line to the intake line is provided in the exhaust gas recirculation line. The EGR valve regulates the amount of exhaust gas to be re-circulated to the intake line by regulating an opening rate of the exhaust gas recirculation line.
Meanwhile, a diesel engine and a compressed natural gas (CNG) engine use an EGR system for different purposes. A diesel engine uses an EGR system for the purpose of reducing the amount of NOx to be generated by chiefly decreasing a combustion temperature within a cylinder. A diesel engine can fulfill exhaust gas regulations only by accurately controlling the amount of exhaust gas to be re-circulated throughout an overall operation range. In contrast, a CNG engine uses EGR chiefly for the purpose of decreasing a temperature of exhaust gas rather than for the purpose of preventing NOx from being generated because the CNG engine emits a considerably lower amount of NOx than a diesel engine. In a CNG engine, when a temperature of exhaust gas is excessively high, durability of parts related to an exhaust line must be high, with the result that a cost of a product is increased. Accordingly, in a CNG engine composed of parts of an exhaust line having relatively low durability, it is necessary to perform control through EGR so that a temperature of exhaust gas can be maintained at a reference value or lower. For this purpose, a CNG engine generally uses a method of additionally providing a universal exhaust gas oxygen (UEGO) sensor in an intake line and performing closed-loop control in order to perform EGR valve control. However, this method is problematic in that the cost is increased due to the addition of the expensive UEGO sensor. Furthermore, the fact that the UEGO sensor is weak to moisture included in exhaust gas supplied through an exhaust gas recirculation line must be taken into account, and thus a problem arises in that a configuration becomes complex in order to ensure durability of EGR valve control.